Many metal oxides such as titanium oxide (TiO.sub.2) are called optical semiconductors since electrons on their surface are made movable relatively freely when they are excited by ultraviolet rays. The optical semiconductor has a photocatalytic function of oxidizing or deoxidizing a substance in contact with a surface of the semiconductor by the electrons which are rendered freely movable by the excitation.
The photocatalytic function is utilized in a daily life to attain an effect of deodorization or sterilization. For example, in order to purify the air in a room of a house or facilities open to general public such as hotels, public buildings and hospitals, the photocatalytic function is given to materials of interiors including ceilings and walls, or the photocatalytic semiconductor is retained in curtains which are often exposed to ultraviolet radiation of the sunlight. Thus, the air which rises to circulate in the room when heated by the sunlight comes into contact with sheets of these materials, so that odors in the room and volatile organic compounds (VOC) contained in adhesives in backings of the interior materials are effectively removed.
The woven fabrics for the interior materials and the curtains, as well as filters in various devices, medical gauze, moistened tissue and nonwoven fabrics such as artificial leather have a large number of gaps between their fibers and thus are multi-surface objects, and hygienic, medical or deodorizing effect can advantageously be obtained by imparting the photocatalytic function to these objects.
These advantageous effects can be achieved on condition that the fiber or fabric itself, which is a base for retaining the photocatalytic semiconductor, should not be decomposed by the photocatalytic function. However, the photocatalytic function is originally a function of decomposing the polymeric organic compounds by the oxidization/deoxidization action of the photocatalytic semiconductor which is excited by irradiation of ultraviolet rays. Therefore, no problem arises if the base retaining the photocatalytic semiconductor is made of an inorganic material such as metallic fibers or glass fibers, but in the case where the base is made of fibers of polymeric organic compound such as natural fibers or synthetic resin fibers, the base itself is decomposed and deteriorated with lapse of time (deterioration by photocatalysis).
Further, polymeric organic compounds are decomposed and deteriorated also by ultraviolet rays (photochemical deterioration). Most of the external ultraviolet rays falling on the base having the photocatalytic semiconductor layer on the surface thereof are absorbed at the photocatalytic semiconductor layer, but the remaining rays reach the base.
As a conventional sheet having the photocatalytic function, there is known a sheet in which particles of white pigment, each retaining particles of photocatalytic semiconductor on its surface, are contained or filled in an air-permeable sheet as proposed in Japanese Laid-Open Patent Publication No. 7-299354. However, this sheet with a unique structure lacks versatility and it is not possible to make a soft material such as gauze by using the sheet.
Japanese Laid-Open Patent Publication No. 7-316342 discloses that synthetic resin containing particles of photocatalytic semiconductor is formed into a sheet for wall materials, floor materials, and various bags. However, when the photocatalytic semiconductor particles are thus kneaded into the base, the photocatalytic function cannot be fully achieved. Also, the technique disclosed in this publication cannot be applied to fabric made of natural fibers which cannot be kneaded.
Further, Japanese Laid-Open Patent Publication No. 8-1010 discloses an adhesive sheet having a layer of fine particles of oxide semiconductor on one surface thereof, an adhesive layer as an intermediate layer, and a separating layer on the other surface thereof. This sheet is intended to use at locations where it is difficult to fix photocatalytic semiconductor particles, such as walls or ceilings, and also cannot be applied to soft material such as gauze.
In the conventional sheets of these types, no consideration is given to prevention or suppression of the deterioration by photocatalysis and the photochemical deterioration. Also, the washability of the sheet for reuse and the use of the sheet in an environment without sufficient ultraviolet radiation, such as in the nighttime or in a dark room, are not taken into account.